1. Field of the Invention
The present invention relates to a toner and a developer for use in electrophotography. In addition, the present invention also relates to an image forming method using the toner, and a toner container containing the toner.
2. Discussion of the Background
In an electrophotographic apparatus or an electrostatic recording apparatus, an electric latent image or a magnetic latent image is visualized with a toner. For example, in electrophotography, an electrostatic latent image formed on a photoreceptor is developed with a toner to form a toner image. The toner image is typically transferred onto a transfer material, and then fixed upon application of heat. Typically, a toner for use in an electrostatic latent image development is a colored particulate material in which a colorant, a charge controlling agent, and other additives are dispersed in a binder resin. Toner manufacturing methods are broadly classified into pulverization methods and polymerization methods.
In a pulverization method, a colorant, a charge controlling agent, an offset-inhibitor, etc. are mixed and melt-kneaded with a thermoplastic resin, and then the mixture is pulverized and classified to prepare toner particles. Pulverized toners typically have properties on a reasonable level, however, materials that can be used for the pulverized toners are limited. For example, the melt-kneaded mixture has to be pulverized and classified using an economically usable apparatus. Therefore, the melt-kneaded mixture has to be brittle. In this case, particles having various particle diameters tend to be produced, i.e., the resultant toner has a broad particle diameter distribution. In order to produce high definition and high gradation images, for example, fine particles having a particle diameter of not greater than 5 μm and coarse particles having a particle diameter of not less than 20 μm have to be removed, resulting in deterioration of the toner yield. In addition, it is difficult to uniformly disperse toner components (such as a colorant and a charge controlling agent) in a thermoplastic resin in the melt-kneading process. When the toner components are insufficiently dispersed in the thermoplastic resin, the resultant toner has poor fluidity, developability, and durability, and cannot produce high quality images.
On the other hand, a dissolution suspension method for preparing toner is known. In the dissolution suspension method, a resin solution in which a resin is dissolved in a solvent is dispersed in an aqueous medium including a dispersing agent or a dispersing auxiliary agent (such as a surfactant and a water-soluble resin), and then the solvent is removed upon application of heat or under reduced pressure to prepare toner particles. However, the toner particles have various particle diameters, i.e., the toner has a broad particle diameter distribution and therefore the classification process is needed.
Japanese Patent No. (hereinafter referred to as JP) 3344214 discloses a dissolution suspension method using a particulate inorganic material (such as a calcium carbonate and a silica) as a dispersion stabilizer to prepare resin particles having a uniform particle diameter. However, in this method, the particulate inorganic material adheres to the resin particles. Even if the resin particles are subjected to the particulate inorganic material removal process, a slight amount of the particulate inorganic material or inorganic ions tend to remain on the surface of the resin particles and deteriorates electric property, thermal property, and chemical stability of the resultant toner.
In attempting to solve this problem, JP 3455523 discloses a dissolution suspension method using a particulate resin (such as a vinyl resin) as a dispersion stabilizer to prepare resin particles (i.e., toner particles) having a uniform particle diameter. However, the particulate resin also tends to adhere to the surface of the toner particles. Since polarity and molecular weight of the particulate resin are different from those of the toner particles, each of the toner particles cannot be fused with each other when the toner is fixed. As a result, interfaces between the toner particles are not homogeneous. In addition, since the toner particles and the particulate resin include different resin components, light is refracted and scattered at interfaces of the toner particles in the toner layer, resulting in deterioration of transparency of the toner layer. Particularly, when the toner is a color toner (for producing a full color image) using a polyester resin having different polarity from and less compatibility with the particulate resin, the toner may not reproduce a native color thereof when layers of the color toners (e.g., yellow toner, magenta toner, cyan toner, etc.) are overlaid. For this reason, it is difficult to produce high quality images having the same quality as high-class printing images.
On the other hand, a toner is required to have good releasability from a heating member such as a heat roller used in a contact heat fixing method. (This property is hereinafter referred to as hot offset resistance.) In a toner prepared by the dissolution suspension method, the hot offset resistance can be improved by using a modified polyester resin formed by a reaction of a precursor of a polyester resin. However, since the modified polyester resin and a main binder resin are different in composition and polarity, these resins are less compatible with each other. As a result, transparency of the toner layer deteriorates and high quality full color images are difficult to be produced.
Besides the hot offset resistance, a toner is required to have low temperature fixability. Conventionally, resins such as styrene-acrylic resins, polyester resins, and epoxy resins have been widely used as a binder resin. Recently, cross-linked polyesters are used as a binder resin because of having good low-temperature fixability.
In attempting to improve both hot offset resistance and low temperature fixability, published examined Japanese Patent Applications Nos. (hereinafter referred to as JP-B) 4-44744 and 7-86699, and JP 305167 have disclosed toners using two kinds of polyester resins having different molecular weight distributions. These toners have relatively well-balanced hot offset resistance and low temperature fixability, compared to conventional toners using one polyester resin. However, the toners have a drawback in that since the two polyester resins having different softening points are mixed in powder states, the mixture has less uniformity, and as a result, a colorant cannot be uniformly dispersed in the toner. If the softening points of these two resins are closer to each other, the colorant dispersibility improves, but the balance between hot offset resistance and low temperature fixability deteriorates.
On the other hand, a toner typically includes a binder resin in an amount of not less than 70%. Since most of the conventional binder resins are made from oil resources, there are concerns of depletion of the oil resources and the global-warming problem caused by discharge of a huge amount of carbon dioxide gas into the air due to heavy consumption of the oil resources. If a binder resin can be synthesized from a plant which grows by utilizing carbon dioxide gas in the air, the carbon dioxide gas can be circulated. Namely, there is a possibility of preventing the global-warming and the depletion of the oil resources. Therefore, polymers derived from plant resources (i.e., biomass) are receiving attention recently.
In attempting to use polymers derived from plant resources as a binder resin, JP 2909873 discloses a toner including a polylactic acid as a binder resin. However, since polylactic acids have ester groups at a higher concentration compared to polyester resins, the polylactic resin has too high a thermal property to serve as a thermoplastic resin when the toner is fixed. In addition, because of having too high a hardness, the polylactic resin cannot be used for pulverized toners.
Published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 9-274335 discloses a toner including a polyester resin formed by a dehydration polycondensation reaction between a lactic acid and an oxycarboxylic acid having 3 or more functional groups. However, since the polyester resin formed by a dehydrate polycondensation reaction between an alcohol group of the lactic acid and carboxyl group of the oxycarboxylic acid has high molecular weight, sharply-melting property and low temperature fixability of the toner deteriorate.
JP-A 2001-166537 discloses a toner including a polylactic acid-type biodegradable resin, and a terpene phenol copolymer as a low-molecular-weight constituent. However, this toner does not have a good combination of low temperature fixability and hot offset resistance.
As mentioned above, a toner using a polylactic acid resin does not still come into practical use.